Free cutting alloys



Patented Jan. 7, 1936 PATENT OFFICEII FREE CUTTING ALLOYS Louis W. Kemp! and Walter A. Dean, Cleveland, Ohio, a'ssignors to Aluminum Company of America, Pittsburgh,

Pennsylvania Pa., a corporation of No Drawing. Application May 3, 1935, Serial No. 19,608

'4 Claims.

This invention relates to aluminum base alloys and it is particularly concerned with those alloys containing silicon as a major alloy component. This application is a continuation in part of our copending application Serial No. 689,880, filed September 18, 1933. claimed herein are claimed in our above-mentioned copending application Serial No. ,689,880 and our copending applications Serial Nos. 19,607, 19,609, 19,610, 19,611, 19,612, 19,613, 19,614, 19,615, and 19,616, filed May 3, 1935.

Aluminum base alloys containing from about 3 to 15 percent of silicon have been widely used in cast, extruded and wrought forms because of such desirable casting characteristics as fluidity and low solidification shrinkage of the molten alloy, combined with a good workability of the solid metal if a wrought article is to be made.

The alloys also possess a low coeflicient of thermal expansion and satisfactory phy'sical properties, particularly the tensile strength and elongation. In spite of these advantageous properties, the field of utility of these alloys has been restricted because of their inherently poor machining quality and consequent increasedcost of finishing articles of these alloys by machining operations such as boring, drilling, shaping. planing, or lathe-cutting. Difilculty in machining is made evident through an excessive cutting tool wearwhich necessitates a frequent resharpening. Poor machinability is also manifested in a rough irregular machined surface and the tendency to form long .chips that may foul the cutting tool or operating parts of the machine. Aluminum-silicon alloys are furthermore quite likely to have hard particles of elementary silicon distributed throughout the alloy matrix which seriously interfere with smooth cutting of the article being machined. These difllculties are most readily apparent in machining operations" conducted on a large scale such as the manufacture of pistons for. internal combustion engines where it has been found desirable to use an ,-expensive cutting tool'material to insure long tool life and a satisfactory machined surface. a

silicon particles and to produce a smooth pleas-- ing surface on the machined article. A further. cbiect'is to accomplish the foregoing ends with-'.

Alloys disclosed but not] out disadvantageously affecting the physical properties of the alloy.

Our invention is predicated upon the discovery that the addition of lead and/or thallium to aluminum-silicon alloys containing from about 3 to 15 percent silicon greatly improves the machining quality of this type of aluminum alloy. The improvement is especially marked in those alloys containing from about 10 to 15 per cent of silicon which have heretofore been particularly diilicult to machine. We have found that the addition of from about 0.05 to 10 per cent of lead and/or thallium to aluminum-silicon alloys imparts a machinability to the alloy which renders it adaptable to all ordinary machining require ments. For the purpose of our invention, lead and thallium are substantially equivalent and areconsidered to constitute a class of elements with respect to their effect upon the machining quality of aluminum-silicon alloys.

The improvement in machining characteristics referred to above may be brought about by the addition of between about 0.05 and 10 per cent of lead or thallium to aluminum base alloys containing from about 3 to 15 percent silicon, but for many applications we prefer, to use between about 1 and 5 per cent of either lead or thallium. The effect on machinability of these added elements is manifested bythe smooth flow of chips from the article being machined, theshort breakable chips that vare produced; the absence of galling or unevenness of cut, resulting in an irregular surface of dull appearance, and the increase in cutting .speed which is possible under such favorable conditions. As an example of an alloy possessing these properties the following composition may be cited: 12 per cent silicon, 4

- per cent lead, balance substantially all aluminum.

Lead and thallium may not only be successfully used in binary aluminum-silicon'alloys, but also in alloys containing copper, magnesium, manganese and the like in addition to the silicon. The action of lead and thallium in improving the machinability of such allows appears to be substantially independent of the other alloying ingredients added to alter the character of the per cent magnesium, the balance being aluminum.

This alloy in cast form may be used for making cylinder heads for certain types of internal combustion engines. Itis desirable that such a casting be readily machinable without diminishing the strength at ordinary or elevated temperatures; We have found that the addition of about 2.25 per cent of lead produces a marked improvement in machining-quality without impairing any of the other desired properties of the alloy.

Lead and thallium may not only be used separately in alloys of the type herein disclosed, but

-'they may be effectively employed in combination in certain applications. The-two elements frequently co-act in an aluminum-silicon alloy to producea degree of machinability not attainedby an equivalent amount of either element alone. When so used in combination the total amount should not exceed 10 per cent and preferably less than 5 per cent is recommended. The relative proportions of the two elements may be varied but approximately equal amounts of them generall'y'give satisfactory results.

For certain applications it has been found desirable to add other elements than lead and thallium whichalso serve to improve the machining quality of the alloy. The elements cadmium and bismuth belong to this class of substances and may advantageously be combined with lead and/or thallium. When vso used, the metals cadmium and bismuth'should be added in quantitles of between about 1.5 and 6 per cent separately or in combination, the total amount in no case exceeding about 6 per cent. An alloyof this type which has been found to have a satisfactory machining quality is one containing about 12 per cent of silicon, 2 per cent of lead, and 2 per cent of cadmium, the balanc'ebeing substantially all aluminum. v

Bismuth likewise may be advantageously employed in combination with lead to produce a readily machinable alloy. From about 1.5 per cent to 6 per cent of bismuth, together with lead in amount as set forth hereinabove, produces a more readily machinable alloy than one in which lead aloneis used. Specifically, a combined total. of lead' and bismuth, say 4 per cent, is moreeffective in this'respect than is 4 per cent of lead or 4 per cent of bismuth alone. We have found that an alloy made up of about 12 per centsilicon, 2 per cent bismuth,and 2 per cent lead, the balance being aluminum, has much more favorable machining properties than the same alloy without the lead and bismuth. The total amount of lead and bismuth should in no case exceed about 10 per cent, and for most purposes a total of less than 7 per cent of lead and bismuth is 5 preferred.

The tensile properties of the alloys herein described are not materially affected by the addition of lead and/or thallium in amounts of less than 5 per cent but there is a decline in strength 10 11' larger quantities are added. However, the machinability of the alloy is somewhat better in such cases which compensates in part for any loss in strength. The tensile properties are likewise afiected if both elements are present and the 15 total amount of lead and thallium exceeds about 5 per cent.

The lead and thallium may be mostconveniently added to the molten aluminum-silicon alloy in solid metallic form. Other methods of making 20 alloy additions practiced by those skilled in the art yield equally satisfactory results. If the amount of lead and thallium to be added exceeds about 1.5 per cent, the melt should 'be heated somewhat above ordinary temperatures and vig- 25 orously stirred to assure a uniform mixture. The method of adding lead and thallium to aluminum and its alloys here referred to is more fully described in copending application, Serial No. 689,- 885, now issued as U. S. Patent No. 1,959,029.

The term aluminum used herein and in the appended claims embraces the usual impurities found in aluminum ingot of commercial grade, or picked up in the course of the usual handling operations incident to ordinary melting practice. 35

The alloys herein disclosed may be subjected to the usual thermal treatments familiar to those skilled in the art for the purpose of improving or altering their physical characteristics.

We claim:

1. An aluminum base alloy containing from about 3 to 15 per cent of silicon, from about 0.05 to 10 per cent of lead and from about 1.5 to 6 per cent of bismuth, the balance being aluminum. 1

2. An aluminum base alloy containing from about 3 to 15 per cent of silicon, from about 1 to 5 per cent of lead and from about 1.5 to 6 per cent of bismuth, the balance being aluminum.

3. An aluminum base alloy containing from about 10 to 15 per cent of silicon, from about 0.05 to 10 percent of lead, and from about 1.5 to 6 per cent of bismuth, the balance being aluminum.

4. An aluminum .base alloy containing from about 10 to 15 per cent of silicon, from about 1 to 5 per cent of lead, and from about 1.5 to 6 per cent of bismuth, the balance being aluminum.

LOUIS W. KEMPF. WAL'I'ER A. DEAN. 

